The present invention relates to a semiconductive rubber roller or, more particularly, to a rubber roller of which the cylindrical rubber layer on and around a conductive core mandrel has semiconductivity and suitable for use in a development unit of a photocopying machine as a development roller or as a toner-transfer roller.
It is known that a so-called semiconductive rubber roller consisting of a core mandrel of an electroconductive material such as metals and a semiconductive rubber layer on and around the core mandrel is used in a development unit of a photocopying machine, which is a device to visualize the electrostatic latent images built up in a photosensitive drum as a latent image carrier, by using a triboelectrically charged toner in the form of a thin layer on the outer surface of a toner carrier of which the surface layer is made from a semiconductive rubber. The semiconductive rubber layer in the semiconductive rubber roller is required to have an adequate electroconductivity, high weatherability to withstand adverse ambient conditions, low rubber hardness and good triboelectric chargeability. In this regard, the semiconductive rubber layer is formed usually from a rubber composition composed of a urethane rubber, NBR or silicone rubber as the base component with admixture of an electroconductivity-imparting agent which is an ionically conductive compound or an electroconductive filler.
As a method for controlling the quantity of electrostatic charges borne on the toner carrier, an article entitled "Contact Charging of Aminosilane-treated Silica Particles" by T. Oguchi, et al. appearing in "Shikizai" (Coloring Materials), volume 56 (9), pages 630-636 (1982) teaches that the quantity of contact charges can be controlled by adequately selecting the kind and amount of the polar groups such as amino and hydroxyl groups to be adsorbed on the surface of the toner particles.
As is known, the chargeability of an organic compound is determined by the electron-accepting and electron-donating behavior of the molecules constituting the compound. According to an article entitled "Capturing Surfaces and Interfaces with Static Electricity" by T. Oguchi appearing in "Hyomen" (Surface), volume 33, No. 3 (1995), the electron-accepting group, i.e. acidic group, is exemplified by nitro, halogen and sulfone groups and an organic compound having these groups introduced into the molecular structure exhibits negative chargeability while the electron-donating group, i.e. basic group, is exemplified by amino and ammonium groups and an organic compound having these groups introduced into the molecular structure exhibits positive chargeability. Japanese Patent Kokai 61-173270 discloses a triboelectrically chargeable blade as an application of the above mentioned information.
It is usual that the conventional semiconductive rubbers for semiconductive rubber rollers based on a urethane rubber or NBR are compounded with various kinds of process oils and softening agents with an object of decreasing the rubber hardness. Since these oily additives added in the rubber necessarily cause the phenomenon of bleeding on the surface of the roller, the surface of a semiconductive rubber roller is usually provided with a protective coating layer from a resinous material such as urethane resins and nylon resins. A problem in such a resin-coated semiconductive rubber roller is that the weatherability of the resinous coating layer is not always very high so that, when the semiconductive rubber roller is kept prolongedly in an atmosphere of high temperature and high humidity, the resinous ingredient in the coating layer is subject to a hydrolysis reaction to cause troubles that the rubber roller adheres to the photosensitive drum or the chargeability behavior of the semiconductive rubber roller is greatly affected depending on the changes in the ambient temperature and humidity in an extreme case. Furthermore, the chargeability characteristic of conventional semiconductive rubber rollers relative to toner particles is more or less dependent on the ambient conditions so that the distribution of electrostatic charging is sometimes very broad resulting in defective development with grayish background or so-called fogging due to local deficiency of electrostatic charging.
On the other hand, a semiconductive rubber roller using a semiconductive silicone rubber has excellent stability and little dependency on the ambient conditions in the chargeability characteristic against negatively charged toner particles with very uniform distribution of charging though with a problem that the trouble of fogging in printing is sometimes caused due to insufficient transfer of the static electricity to the toner particles but this measure is not very satisfactory because of the frequent occurrence of fogging in the initial stages under adverse ambient conditions of high temperature and high humidity.